What is the flexibilizer of adhesive ?

Flexibilizers are a single-functional group or multi-functional group compounds, which can react with main polymer as as part of curing system. For example, unmodified epoxy resin has serious brittleness after curing, which may lead to fracture spreading in adhesive layer when the joint needs to stand pressure.In order to improve the brittleness of epoxy resin, impact resistance and peel strength, flexibilizers are applied to adhesive. Flexibilizers can enhance shear strength, peel strength, low temperature performance and flexibility.

The frequently used flexibilizers of adhesivesare below.

① Unsaturated polyester resin
③Polyamide resin
④Acetal resin
⑤Polysulfone resin
⑥ Polyurethane Resin 

Flexibilizer are divided into two types including activated flexibilizer and inactivated flexibilizer. Inactivated flexibilizer will not participate in curing reaction, staying in adhesive layer as free state. Generally, the usage of epoxy resin is about 10%20%. The frequently-used flexibilizers are dibutyl phthalate ( DBP), dioctyl phthalate( DOP) and triphenyl phosphite. Adding inactivated flexibilizer can enhance the flexibility of cured epoxy resin, while the thermal distorsion temperature has been decreased. So it is not an ideal method to improve flexibility.

Activated flexibilizerparticipate in curing reaction with more obvious effects compared with inactivated flexibilizer. The frequently-used flexibilizers are liquid polysulfide rubber, liquid nitrile rubber, liquid CTBN rubber, polyurethane resin and low molecular weightpolyamide resin.

Liquid polysulfide rubber is a viscous liquid with low molecular weight, which has properties of excellent elasticity and adhesion, oil resistance and chemical resistance. When the liquid polysulfide rubber is mixed with epoxy rein, the terminal carboxyl will react with epoxy, which can create better flexibility for epoxy resin.

It is easy for liquid nitrile rubber to process and mix with epoxy rein. The common molecules of liquid nitrile rubber can not react with epoxy resin without activated group. But the terminal carboxyl will react with epoxy resin. The experiment proves that the carboxyl in liquid nitrile rubber is more effective than that in liquid polysulfide rubber. Furthermore, the polyurethane modified epoxy resin are used as the adhesive in low temperature.The polyurethane type can be divided intopolyester type and polyether polyurethanewith good impact resistance in low temperature.

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